PORSCHE says no, it is not possible nor allowed to machine them, once worn or damaged they need to be replaced no if or but.
They have a life expectancy of 300000km on road vehicles and are usually not used on race cars as they chip easily when not careful during wheel changes. All GT3 cup cars in NZ have the steel discs fitted.
If you use a diamond grinding tool similar to the flywheel grinding as mentioned earlier it may work but chances are that they will shatter.
Just checked, new ones for the front are around NZ$11000+ GST and for the rear around NZ$9000 + GST, these prices are retail. Pads for the front are NZ$ 1500. Alternatively you can use steel discs on the front with the 6 piston calipers but for the rear you need the calipers too as the ceramic ones are different
